Download Effects of Pathogens On Plant Physiology

Effects of Pathogens
On Plant Physiology
• Photosynthesis
• Translocation
– Water
– Nutrients
• Respiration
• Membrane Permeability
• Transcription and Translation
Leaf Spot, Blight, Other
Destruction of Leaf Tissue = 
Photosynthesis (P/S)
6CO2 + 6H2O  C6H12O6 + 6O2
Northern Leaf Blight on Corn caused by the fungus
Exserohilum turcicum, previously called
Helminthorsporium turcicum. The fungus overwinters as
mycelium and spores in corn residue. Spores are
dispersed by wind and splashing water. Disease
development is favored by extended periods of lead
wetness and moderate temperatures.
As lesions of Phomopsis leaf blight (caused by the fungus Phomopsis
obscurins) mature they are delimited by veins and develop a
distinct V-shape.
Photosynthesis
• Toxins May Inhibit P/S Enzymes
• Decrease in Chlorophyll Content
• Chlorosis
• Closed Stomata
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Xylem
• Water and Minerals Absorbed by Roots
• Translocated through the Xylem
Vessels of the Stem
• Into Vascular Bundles of Petioles and
Leaf Veins
• Enter the Leaf Cells
Pathogens Interfere with
Normal Translocation Process
• Flow of Nutrients or Water Blocked

• Disease in Affected Cells/Tissues

• Inability to Carry Out Functions 
• Shortage of their Products 
• Disease in Entire Plant
2. Xylem Gets Destroyed
– Rot or Canker Pathogens
– Gall Formation
3. Xylem Gets Clogged
– Growth of Vascular Wilt
Pathogens
– Pathogen Secretions
– Secretions of Tylose by Plant
in Response to the Pathogen
Xylem
• Most of the Water Evaporates and
Diffuses into the Atmosphere via the
Stomata
• Results in Water Tension in the Xylem
• Pulls the Water Column Upward
• Stimulates additional Uptake
How Can Pathogens Impair
Translocation of Water and Inorganic
Nutrients?
1. Root Damage
– Damping-Off
Fungi
– Root-Rot Fungi,
Bacteria
– Most Nematodes
– Some Viruses
• Mechanisms of
Damage
– Cause Direct
Injury to Roots
– Inhibit Root Hair
Production
– Alter
Permeability of
Root Cells
Excessive Transpiration Occurs
when Pathogens Damage Leaves,
Stomata
• Leaf Cuticle Protects against Excessive
Water Loss
– Rusts, Mildews and Apple Scab Destroy
Considerable Portion of Cuticle and Epidermis
– Loss of Turgor and Wilting of Leaves
– Suction Forces may Lead to Collapse or
Dysfunction of Underlying Vessels
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Phloem
 Carbohydrates Produced in Leaf Cells
Move through Plasmodesmata into
Adjoining Phloem Elements
 Move down Phloem Sieve Tubes
 Move into Nonphotosynthetic Cells or
into Storage Organs
 Nutrients Are Removed from
‘Circulation’
Obligate Fungal Parasites (eg: Rust and
Mildew)
– Respiration Increases
– P/S Decreases
• However, Photosynthetic Products Accumulate in
Invaded Areas
• Apparently Nutrients from Uninfected Areas Are
Translocated toward the Infected Areas
How Can Pathogens Impair
Translocation in the Phloem?
• Movement of Substances from Leaf Cells to
the Phloem
• Translocation through Phloem Elements
• Movement from Phloem into Cells that will
Use the Nutrients
Viruses
– Viruses that Cause Leaf-Curling and some
Yellows Diseases Cause Death of Phloem
– Some Viruses Inhibit Enzymes that Break Down
Starch into Smaller, Translocatable Molecules
Plant Respiration
• Cells Break Down Carbohydrates to CO2 and H2O in
Series of Steps
C6H12O6 + 6O2  6CO2 + 6 H2O
• Energy Released when the Carbohydrate Bonds
Are Broken Is Used for Cellular ‘Work’
–
–
–
–
–
Accumulation and Mobilization of Compounds
Synthesis of Proteins
Activation of Enzymes
Cell Growth and Division
Defense Reactions
• One of first Functions Affected when Plants Are
Infected by Pathogens
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Disease =  Respiration
• Shortly after Infection—by the Time the Symptoms
Are Visible
– Rate Continues to Rise during Multiplication and
Sporulation of the Pathogen
– Declines to Normal or below-Normal Levels
– Rate Increases more Rapidly in Resistant Varieties
• Increased Respiration Depletes Plant’s Reserves
• Changes Metabolism
• Increased Activity of Enzymes
• Increased Synthesis, Accumulation and Oxidation of Phenolics
Cell Membranes
• Double Layer of Lipid Molecules
• Protein Molecules Embedded
– Parts usually Protrude on One or
both Sides of Lipid Bilayer
• Membranes Are Selective Barriers
– Allow Entry of Substances the Cell Needs
– Inhibit Passage out of the Cell
• Cell Wall Keeps Large Molecules away from Membrane
• Small Water-Soluble Molecules such as Ions, Sugars and
Amino Acids Flow through or Are Pumped through
Membrane Channels (Proteins)
• Disruption of the Cell Membrane Alters Permeability
Membrane Permeability Is often the
First Detectable Response of Cells
to Infection
• Leakage of Electrolytes
• Unknown if Membrane Leakage Is 1 or 2 Effect
– If Direct Attack, Pathogens probably Use One of the
following Strategies
• Stimulate Membrane-Bound Enzymes that Pump H+ in and K+
out
• Interfere with Maintenance and Repair of Membrane’s Fluid
Film
• Degrade the Lipid or Protein Components of Membrane
– Uncontrollable Loss of Useful Substances and Entry of Harmful
Substances
Transcription and Translation
• Transcription
– Copying Coding on DNA onto Messenger RNA
• Translation
– Use of Messenger RNA as a Template to Produce
Proteins
• Disturbances of any of the Processes may Affect
Expression of Genes and Cause Drastic,
Unfavorable Changes in Structure and Function of
Affected Cells
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Effect on Transcription
• Some Pathogens Change Composition, Structure or
Function of Chromatin Associated with Cell DNA
– Virus Uses Host Cell’s Nucleotides and Machinery to Make Its own
RNA
• Sometimes See  Activity of Enzymes that Break Down
RNA
– Pathogen may Be Forcing Plant to Produce New Kinds of Enzymes
not Produced in Healthy Plants
• Higher Levels of RNA in Infected Plants, especially
Resistant Ones
– Probably Due to Increased Synthesis of Substances Involved in
Defense Mechanisms
Effect on Translation
• Higher Energy Needs
– Increased Activity in Enzymes Associated with
Respiration
• Increased Production of Phenolics or Oxidation of
Phenolic Compounds
– Used in Defense Reactions
• Resistant Plants Have  Protein Synthesis in First
few Minutes of Infection
– Levels Remain High up to 2 to 20 Hours after
Inoculation
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